Examples of scanning probe microscopes are described in WO 2008/071013 and WO 2008/006229. They have a probe forming a tip to be moved along a sample. The probe is continuously oscillated by means of an oscillating voltage applied over a piezoelectric actuator.
A typical design for driving the piezoelectric actuator Q in a conventional device is shown in FIG. 1. As can be seen, an oscillating voltage is applied over the actuator Q and the current flowing through the actuator Q is amplified by means of an amplifier A. The output of amplifier A is a measure for the mechanical amplitude of the oscillation of the actuator. In order to keep this mechanical amplitude constant, a feedback loop is required that strives to keep the amplitude of the output of amplifier A constant by appropriately controlling the oscillator. Since the piezoelectric actuators are driven in a resonance and a high quality factor of this resonance is of special interest to enhance the sensitivity to frequency shifts, the control of the amplitude is quite demanding with respect to the dynamic range of the amplitude of the excitation signal and with respect to the controller design.